It is a common practice in interventive radiology to identify a target as seen on computerized tomographic (CT) images and then seek that target through the skin by means of a probe. This probe might be a biopsy needle, drainage catheter, radionuclide catheter, radio frequency or microwave electrode, drug delivery catheter, etc. At the present time, the radiologists use essentially a free hand method, estimating the depth of the target from a point on the skin and also the angulation of the probe from that point to the target. For years a similar problem has been approached for the case of the brain. Here the situation is much simipler in that the skull represents a rigid anchoring platform onto which head rings and localizer systems can be permanently affixed. In this way exact referencing and stereotaxic direction of probes can be achieved in the brain. The body is a different matter. It has no rigid anchoring points and is prone to moving with respiration and patient movement. Respiratory gating and patient immobilization help, but still the problem is far more difficult than the head.
A simple means is needed for the radiologist to identify specific surface points on the skin that can be correlated with corresponding points on the CT image. The means of doing this must be straight forward and unambiguous. It also must circumvent the problem of body motion with respiration and patient movement. The means must be simple to apply and easy to understand and preferably give quantitative coordinates to the operator with a minumum of analysis.
The invention described herein is such a means. For the first time it provides a way to find reference points near or on the surface of the body of the patient over a wide area and over the curved surface of the body itself. By means of its lattice construction, which we will sometimes refer to as a ladder localizer, curvilinear coordinates over the body's surface can be identified discretely and orthogonal components may also be determined to provide a unique position on the patient's skin. Hereafter, we will refer to the principle axes of the scanner, which approximate the axes of the patient's body, as the z axis or the axial direction perpendicular to the scan slice, and the x and axes which together represent a Cartesian coordinate system as shown in FIG. 1. Thus we refer to an axial slice as one which is parallel to the x, y plane, a sagittal slice as one being parallel to the y, z plane, and a coronal slice as one being parallel to the x, z plane. These are essentially CT machine coordinate planes.
There has been very little prior art in such localizer systems, and none with the objectives that we have in mind. R. Brown, et al. (Investigative Radiology, Volume 14, Page 300, July, 1979); indicates a localizer system for use in brain stereotaxy. It is a rigid structure which can be attached rigidly to the skull by means of an intermediate head ring. It has N-type structures on its localizer which enables a point on the diagonal only to be determined. You cannot get a rod coordinate unless the localizer is aligned with the scanner beforehand. Brown's localizer is not designed to be placed near the skin for localizing a point on or near the skin. It is designed for localization of a plane which thereafter can be used to determine targets within the body. It teaches no use of scales affixed to the localizer for means of digitally finding a physical point on a rod or a diagonal. Brown's localizer is not adapted for placement close to or on the skin for the purposes described in our invention. It is also not used as part of a localizer where added rods beyond the two rods off the N-structure are used in order to determine the CT plane intersection of the added rods as further points near the skin.
Hammerschlag et al. Computed Tomography of the Spinal Canal, Radiology, Volume 121, Page 361, 1976 describe a skin marking system composed of polyethylene angiographic catheters, each of which differ in length. This system of catheters is designed to help determine the level or z axis (axial position) of the CT plane. The degree to which you can define the plane is no better than the quantum length differences between adjacent catheters, in their case being 1 cm. Thus, it is a discrete means, not a continuous means as in our invention, of z identification of the CT plane. The diagonal element in our invention provides a continuous means for plane identification and a means of determination of the plane intersection with both the diagonal and rod elements. Furthermore, the Hammerschlag system enables visualization of their catheters only on one side of the last catheter which is intersected, not on the entire array. This is again different from our invention and a limitation, whereas our rod elements are described as of equal length, all of which appear in the same scan. Hammerschlag describes no scale means on their set of catheters for quantitative z identification nor do they describe a diagonal element to give continuous plane and point localization determination. Thus, the Hammerschlag approach teaches only a crude means of achieving a level or z position of the plane which is unsuitable for the precise localization of a skin point for entry in a surgical procedure as we describe here.